Cathode construction

ABSTRACT

Structures and methods are disclosed for accelerating the warmup time of an electron discharge device having an indirectly heated cathode either by coating the cathode with a high heat absorption material or by increasing the heat radiation properties of the heater.

O Unlted States Patent [151 3,662,21 1 Millis 1 May 9, 1972 [54] CATHODECONSTRUCTION 1,872,359 8 1932 Sutherlin ..313/326 x [72] Inventor:Walter T. Mlllis, Owensboro, Ky. OTHER PUBLICATIONS [73] Assignee:General Electric Company Kohl, Materials and Techniques for ElectronTubes" Reinh ld P bl h' C NY. 1960, 557- 63, K 6 221 Filed: Jan. 15,1970 3 mg 0 pgs 5 T 565 21 A 1. N .1 4,167 1 pp 0 PrimaryE.\'ami11er--R0y Lake Remed .s Application D AssistantE.\'aminerLawrence .l. Dahl Attorney-Nathan J. Cornfeld, O, B. Waddel,John P. Taylor [63] Continuation of Ser. No. 95,932, Mar. 15, 1961,abanand Frank L Neuhauser doned.

[57] ABSTRACT [52] US. Cl ..313/340,3l3/45l:u3); /13:: Structures andmethods are disclosed for accelerafing the [5!] lnt.Cl. warmmp time ofan electron discharge device having an [58] Fleld of Search ..313/45,340, 326; 29/2514, directly heated cathode either y Coating the Cathodewith a 29/2517 high heat absorption material or by increasing the heatradiation properties of the heater. [56] References Cited 5 Claims, 4Drawing Figures PATENTEDIMY 9 I972 FIG. 2

INVENTOR WALTER T MILLIS ATTO NEY CATHODE CONSTRUCTION This applicationis a continuation of application Ser. No. 95,932, filed Mar. 15, 1961,and now abandoned.

This invention relates to electron discharge devices, and moreparticularly to indirectly heated cathodes utilizable in such devices.

Electron discharge devices are being used in applications in whichextremely small warm-up times, so called, are required and while it hasbeen recognized heretofore that reduction in warm-up time can beefiected by arrangements which reduce the temperature differentialbetween the heater and the cathode during the warm-up period, suchexpedients as have been proposed have not resulted in sufficientlyreduced warmup times to meet present requirements.

Constructions are known which would reduce the mass of the cathode, thusto reduce the time required for the cathode to reach effectiveelectron-emission temperature, the socalled warm-up" time. However, themass of the cathode can be reduced only to a point at which itsstructural strength becomes so reduced as to adversely affect itsrigidity. The corresponding warm-up time value has not been found to beadequate for many applications.

It is, therefore, a principal object of my invention to provide anindirectly heated cathode of improved thermal efficiency.

It is another object of my invention to provide a high thermalefficiency cathode of the indirectly heated type characterized in thatthe warm-up time therefor is reduced to a minimal value.

In accordance with the present invention, substantial reduction inwarm-up time is accomplished by increasing the heatradiating property ofthe heater or by increasing the heat-absorption characteristic of thecathode or by the combination of increased heat radiation by the heaterand increased heat absorption by the cathode. Thus, the heater,according to the present invention, is formed with an outer surfacecoating which may be of a carbonaceous material overlying the insulativecovering of the heater wire, or the coating may be of particulated orfinely divided tungsten with similar results.

Further, in accordance with the present invention, the surface of thecathode adjacent the heater is formed with a highheat absorbing materialwhich may be in the form of a coating of carbonaceous material suitablyafiixed to the said cathode surface, or the coating may be of a suitableoxide, say of chromium.

Still further, in accordance with the present invention, a low warm-uptime heater-cathode assembly is provided, characterized in that thetemperature differential between the heater and the cathode is reducedto a relatively low value, which assembly comprises a heater having anouter coating of highheat emissivity in combination with a cathodehaving highheat absorption characteristics.

My invention possesses other objects and advantageous features, some ofwhich together with those described above, will be set forth in thefollowing description and accompanying drawing in which:

FIG. 1 is an elevational view, with parts broken away, illustrating anelectron discharge device embodying the features of this invention;

FIG. 2 is an enlarged view, partly in cross-section, of a heater elementusable with the cathode of the electron discharge device of FIG. 1;

FIG. 3 is an enlarged view, partly in cross-section, of the cathode; and

FIG. 4 is a fragmentary view in cross-section of the assembledheater-cathode assembly in accordance with this invention.

Referring now to the drawing, there is illustrated in FIG. 1 an electrondischarge device 11 which may comprise an evacuated envelope 13, withinwhich is supported a cage assembly including coaxially disposedelectrodes which, in the illustrated embodiment, may include a cathode15, a grid 17 and an anode 19. The cathode, grid and anode elements aresupported between top and bottom insulative spacer elements or micas 21and 23, respectively, the cage being supported from the stem 25 of theenvelope by stem supports as at 27. Connections from the electrodes tothe stem supports may be efiected in the usual manner and affordconnections to the exterior pins 29.

While the invention is disclosed herein as embodied in a triode type ofelectron discharge device, it will, of course, be recognized that thistype is exemplary only and that the features of the invention mayreadily be applied to other types of electron discharge devices, as forexample, diodes, tetrodes, pentodes, et cetera, which in usual formsemploy cathodes of generally cylindrical form and on the outer surfacesof which are deposited suitable electron emissive material which, uponbeing heated, are adapted to provide copious supplies of electrons.

Other types of electron discharge devices in which the features of thepresent invention may also be applied, include the planar-electrodedevices wherein the cathode may be in the form of a substantially planardisk or, sometimes, a dished or cup-shaped member having on one surfacethereof a coating of electron emissive material, the opposite surface ofwhich is adapted to be heated by a heater in next adjacent spatialrelation thereto.

The cathode 15, in accordance with the invention, may comprise agenerally cylindrical, tubular sleeve 31, within which is disposed aheater 33 which, in turn, is adapted to be supplied with suitableheating current from a source (not shown). The heater 33, as shown moreclearly in FIG. 2, may comprise a filamentary conductor 35 which may beof tungsten or other suitable refractory metal, which is coated with aninsulative coating 37 which may be of a conventional variety consistingof a mixture of aluminum oxide and aluminum nitrate, or any othersuitable insulative composition which may be applied to the conductor35, as by drag coating, spray coating, or by any suitable cataphoreticprocess. Overlying the insulative coating 37, and for enhancing the heatradiating properties of the heater, 1 provide an electricallyconductive, high-heat emissive coating 39 which comprises finely dividedcarbon in the form of lamp black or micronized graphite. To apply theheat-emissive coating 39, a quantity of finely divided carbon may beintimately mixed with a mixture of aluminum oxide and aluminum nitrateof the kind used for forming the insulative coating 37 and inproportions ranging from about 1 per cent to about 20 per cent carbon,by weight, and in a suitable vehicle for use with a drag-coatingapparatus such as that disclosed and claimed in US. Pat. No. 2,943,598to W. H. Newton, patented July 5, 1960. The heat-emissive coating mayalso be applied cataphoretically or it may be applied by spraying, inwhich case the carbonaceous mixture would be suspended in a suitablebinder such as nitrocellulose.

The heater resulting from the just-described processing is characterizedby a relatively blackened exterior appearance and provides a substantialreduction in time required to transfer heat from the conductor 35 to thecathode sleeve 31.

In another embodiment of my invention, a blackened outermost coating 39is provided by mixing, with the aluminum oxide and aluminum nitrate,finely divided or pulverized tungsten instead of the carbon of theforegoing example, in proportions of from about 1 per cent to about 40per cent tungsten, by weight, and applied to the insulative coating 37as by drag coating, spray coating or cataphoretically, to provide acoating having the desired blackened high-emissive exterior.

It will be noted that the heater constructions described above not onlyprovide the desired high-heat transfer characteristics but also operateto eliminate a defect found in heatercathodes heretofore employed.Conventional heater-cathode arrangements used in electron dischargedevices which are employed in tuned-circuit arrangements have been foundto present a heater-to-cathode electrostatic capacitance which mayactually form part of the tuned circuit. In such circumstances, relativemovement of the heater and cathode resulting, say from mechanical shockor other vibration-producing disturbances causes undesired variations inthe heatercathode capacitance which, in turn, causes unwanted variationsin the tuned-circuit characteristics. It will be seen, therefore, thatin the heaters as above described, the outer coating is electricallyconductive and when the heater is snugly fitted within the bore of acathode sleeve, an electrical connection between the outer coating ofthe heater and the cathode sleeve is effected. Accordingly, the outercoating of the heater and the sleeve are at the same potential, thusrendering the capacitance between the cathode sleeve and the heaterconstant, independent of any relative movement therebetween. Thus,undesired capacitance variations between heater and cathode and theunwanted variations in the tuned-circuit characteristics noted above areeliminated.

In accordance with another feature of this invention, the cathode 31, asshown more clearly in FIG. 3, may be a tubular sleeve that is renderedhighly heat-absorbent by providing a darkened interior coating 41thereon. Such darkened coating may be provided by cladding on the innersurface of the sleeve a relatively thin layer of aluminum. The claddingis preferably perfonned prior to formation of the sleeve by a pressurecladding operation in which flat stock cathode nickel material andaluminum sheet material are tightly pressed together in sheet form underhigh mechanical pressure conditions. The composite metal is thereafterformed into tubular sleeve members interiorly clad with aluminum, andwhich may be provided with butt, lock, or lap seams, as desired, and cutto any desired length. The exterior surface of the sleeve is coated inany conventional manner with an electron-emissive material 43. Inoperation, at the normal operating temperature of the cathode, thealuminum cladding is converted to an aluminumnickel reaction productwhich presents a blackened interior surface having a high-heatabsorption quality.

Another arrangement for providing the relatively darkened high-heatabsorption coating 41 for the cathode sleeve 31 is to carbonize theinner surface of the sleeve. Such a carbonized coating may be applied ina manner similar to that employed in applying the finely divided carbonto the outer insulative coating 37 of the heater wire 35 describedabove. Or, the carbonized coating may be applied by preliminarilymasking one surface of the nickel sleeve material and applying acarbonized coating to the other in accordance with a method describedand claimed in the patent of Perl, U.S. Pat. No. 2,82 l ,496 patentedJan. 28, 1958 and assigned to the assignee of the present application.

In accordance with another feature of the invention, a highheatabsorption coating is formed on the inner surface of the cathode sleeveby plating the cathode nickel sheet material, prior to forming thematerial into sleeves, with a relatively thin layer of chromium. In oneoperative embodiment, a thickness of chromium of about 0.000] inch wasfound to provide, when oxidized in a wet hydrogen atmosphere at atemperature of approximately 700 C., a blackened oxide of chromiumsurface that was highly heat absorbent. The firing of the chromium plateat temperatures above 700 C. and up to about 1,100" C. producedequivalent high-heat absorbent surfaces in times substantially less thanthat described above for the firing at 700 C.

The chrome oxide layer has been found to be extremely stable at thenormal operating temperatures of electron discharge devices and issufficiently flexible to permit forming the sheet material into tubes ofrelatively small diameter without fracture or peeling of the oxide. Itwill be noted that during the firing operation described above, thenickel surface of the sleeve material is not adversely affected, thissurface retaining its bright appearance, probably because nickel oxidesare extremely unstable at the firing temperatures so that such oxidesthat may be formed are readily reduced down leaving a bright, highlyreflective nickel surface suitable as the base for electron-emissivematerial.

In still another form of the invention, the heat absorptive surface 41is formed on the interior coating of the sleeve 31 by cladding thecathode nickel alloy material, while in sheet form, with a coating oflnconel," which is a brand of chromenickel-iron alloy available from theInternational Nickel Company and having approximately 15 per cent, byweight, chromium in an alloy containing components in the followingapproximate proportions:

Nickel 77.0% Chromium 15.0% Iron 7.0% Copper 0.2% Manganese 0.25%Silicon 0.25% Carbon 0.06% Sulphur 0.007%

An alloy having similar composition and as effective as theabove-mentioned "Inconel is obtainable from the Driver- Harris Companyunder the trademark "Nirex.

I have found that a cladding or pressure bond of the highchromiumcontent alloy onto standard cathode alloys, as, for example, No. 225cathode nickel alloy or No. 449 cathode nickel alloy in a ratio ofapproximately per cent chrome alloy to 20 per cent cathode alloy, yieldsa well-blackened high-heat absorptive chromic oxide surface when thecomposite metal is fired in a wet hydrogen atmosphere at a temperatureof about l,000 C. for approximately 10 minutes. The operation ofconverting the chrome alloy to form a dark chrome oxide is atime-temperature process so that if the firing is carried out attemperatures above the noted 1,000 (3., the time cycling can becorrespondingly less than the indicated 10 minutes. Again, the cathodenickel alloy component of the composite material is virtuallyunafi'ected by the firing operation, because any nickel oxide that maybe produced is unstable and reduced down readily at the firingtemperature.

Electron discharge devices constructed in accordance with certain of thefeatures of the present invention have demonstrated a substantialreduction in warm-up time resulting from the high-heat transfercharacteristics provided thereby. In a series of tests conducted withcommercially available types and utilizing a conventional heater, e.g.,one not coated with the externally blackened high-heat emissive materialbut used to heat a cathode sleeve having a darkened interior surface ofchromic oxide, the novel cathode assembly demonstrated reductions inwarm-up time of approximately 30 to 38 per cent as compared to cathodesof conventional construction. Thus, with cathode sleeves having a wallthickness of 0.0035 inch, an average warm-up time of 31 seconds isobserved with a conventional cathode alloy sleeve. This time was reducedto a value of 19 seconds when a cathode sleeve in the form of thecomposite metal of this invention is used in which the ratio of cathodenickel alloy to lnconel" was 2 0 to 80. In another embodiment in whichthe cathode sleeve with a wall thickness of 0.0021 inch was used, thewarm-up time of approximately 23 seconds for a conventional cathodealloy sleeve was reduced to approximately 16 seconds when a cathodesleeve of cathode alloy clad with Inconel" in the ratio of 20 to 80 wasused. In both embodiments, the chromium-alloy cladding was oxidized,prior to formation of the cathodes, in accordance with theabove-described process.

Further reduction in warm-up time can be obtained through the use of aheater-cathode assembly as shown in FIG. 4, wherein a high-heatradiating heater 33 of the type described above is employed to heat ahigh-heat absorptive cathode sleeve 31.

There has thus been described arrangements for reducing the warm-up timeof electron discharge devices by increasing the heat transfercharacteristic between the heater and the cathode of an indirectlyheated cathode assembly. While the invention has been described inconnection with cathode constructions utilizing substantiallycylindrical sleeve members, it will be understood, as noted above, thatthe features of the invention are not limited to employment in suchcathode constructions, it being apparent that the heat radiating andheat absorptive features can readily be applied to relatively planar andcup-shaped or dished cathodes, the side thereof adjacent the heaterbeing blackened in the manner and by the means described above anddisposed in next adjacent relation to a heater of which the insulativecoating is provided with a black high-heat emissive material.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A cathode support for an indirectly heated cathode for electrondischarge devices, comprising a base element of cathode nickel alloy,and a cladding on one surface of said element, said cladding consistingof a selectively oxidized alloy comprising chromium, nickel, and ironhaving an oxide of chromium thereon to provide a relatively highheat-absorptive means, said element being adapted to support on anopposed surface thereof an electron-emissive material.

2. The cathode support as defined in claim I wherein said base elementis a tubular member having electron-emissive material on the outersurface thereof and wherein the chromium, nickel, and iron alloy furthercomprises an oxide of chromium on the inner surface of said tubularmember as said heat-absorbing means.

3. A process for forming a cathode support for an indirectly heatedcathode for electron discharge devices comprising:

a. cladding a cathode nickel sheet with a chrome-nickeliron alloy; b.forming the clad sheet into a cathode support; and c. selectivelyoxidizing in a wet hydrogen atmosphere the chrome metal portion of saidalloy on said support at an elevated temperature to form an oxide ofchrome on said alloy, said wet hydrogen atmosphere inhibiting theoxidation of nickel. 4. The process of claim 3 wherein said support isselectively oxidized at a temperature of about 1,000" C.

5. A process for forming a cathode support for an indirectly heatedcathode for electron discharge devices comprising:

a. cladding a cathode nickel sheet with a chrome-nickeliron alloy b.forming the clad sheet into a cathode support; and c. selectivelyoxidizing the chrome metal portion of said alloy on said support at anelevated temperature to form an oxide of chrome on said alloy.

3, 662 2ll Dated Max 9, 1972 Patent No.

nv (s) Walter T. Millis It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Delete Claim 5, Column 6, Lines 12-19.

Signed and sealed this 12th day of September 1972.

(SEAL) Attest:

EDWARD M.FLETCI-ER,JR. ROBERT GOTI'SCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 w us, GOVERNMENT PRINTING OFFICE: 19690-365-334 FORM PO-105O (10-69)

1. A cathode support for an indirectly heated cathode for electron discharge devices, comprising a base element of cathode nickel alloy, and a cladding on one surface of said element, said cladding consisting of a selectively oxidized alloy comprising chromium, nickel, and iron having an oxide of chromium thereon to provide a relatively high heat-absorptive means, said element being adapted to support on an opposed surface thereof an electron-emissive material.
 2. The cathode support as defined in claim 1 wherein said base element is a tubular member having electron-emissive material on the outer surface thereof and wherein the chromium, nickel, and iron alloy further comprises an oxide of chromium on the inner surface of said tubular member as said heat-absorbing means.
 3. A process for forming a cathode support for an indirectly heated cathode for electron discharge devices comprising: a. cladding a cathode nickel sheet with a chrome-nickel-iron alloy; b. forming the clad sheet into a cathode support; and c. selectively oxidizing in a wet hydrogen atmosphere the chrome metal portion of said alloy on said support at an elevated temperature to form an oxide of chrome on said alloy, said wet hydrogen atmosphere inhibiting the oxidation of nickel.
 4. The process of claim 3 wherein said support is selectively oxidized at a temperature of about 1,000* C.
 5. A process for forming a cathode support for an indirectly heated cathode for electron discharge devices comprising: a. cladding a cathode nickel sheet with a chrome-nickel-iron alloy b. forming the clad sheet into a cathode support; and c. selectively oxidizing the chrome metal portion of said alloy on said support at an elevated temperature to form an oxide of chrome on said alloy. 